Using An LED Die To Measure Temperature Inside Silicone That Encapsulates An LED Array

ABSTRACT

A light-emitting diode (LED) device includes first and second LED dies with the same structure and that are both encapsulated by the same silicone layer. The first LED is supplied with sufficient drive current to illuminate the LED. Control circuitry supplies the second LED with a constant current, determines the voltage across the second LED, and calculates the temperature of the second LED based on the voltage across the second LED. The constant current has a maximum magnitude that never exceeds the maximum magnitude of the drive current. The LED device is able to calculate the temperature of a diode with a gallium-nitride layer (GaN or GaInN) that is receiving a large drive current and emitting blue light by determining the voltage across an adjacent similar diode with a gallium-nitride layer through which a small constant current is flowing. Preferably, the band gap of the LEDs exceeds two electron volts.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.13/930,672 entitled “Using An LED Die To Measure Temperature InsideSilicone That Encapsulates An LED Array,” now U.S. Pat. No. ______,filed on Jun. 28, 2013, the subject matter of which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates generally to packaging for light-emittingdiodes, and more particularly, to a method of determining thetemperature inside a silicone layer that encapsulates an array of LEDdies.

BACKGROUND INFORMATION

Light emitting diodes (LEDs) are an important class of solid-statedevices that convert electric energy into light. Improvements in thesedevices have resulted in their use as light sources replacingconventional incandescent and fluorescent light fixtures. The energyconversion efficiency of LEDs now approaches the level attained byfluorescent light fixtures and promises to exceed even theseefficiencies. Moreover, LEDs have significantly longer lifetimes thanboth incandescent bulbs and fluorescent tubes. However, the usefullifetime of LEDs is significantly reduced if the operating temperatureexceeds certain limits.

The operating environment of an LED light source is typically hot, andoverheating must be controlled in order to extend the operating life ofthe light source. The high operating temperatures of commercial whiteLED light sources result primarily from two factors. First, the phosphorthat converts blue light from the LED dies into longer wavelength lightgenerates heat. Thin layers of Group III nitrides, such as galliumnitrides (GaN or gallium indium nitride GaInN), are used to produce LEDsfor general commercial lighting applications. For example, thinepitaxial layers of gallium nitrides are grown on sapphire substrates(Al₂O₃). Light is emitted from the epitaxial layers sandwiched betweenoppositely doped layers when a voltage is applied across the dopedlayers. Gallium-nitride LED dies (GaN or GaInN) emit blue light having awavelength in a range from 430 nanometers to 460 nanometers. A phosphorcoating then absorbs some of the emitted blue light and fluoresces toemit light with longer wavelengths so that the overall LED device emitslight with a wider range of wavelengths, which is perceived as “white”light by a human observer. The phosphor does not convert all of the bluelight to longer wavelength light, but rather converts much of the bluelight to heat.

Second, a single LED die produces too little light to be used as areplacement for a conventional light source in most applications. Hence,a replacement light source must include a large number of individual LEDdies. The large number of LED dies that are packaged in close proximityto one another under a transparent carrier material that containsphosphor particles results in a large amount of heat generated within asmall volume. The temperature under the transparent carrier materialrises when the large amount of heat generated by the many LED diescannot be conducted fast enough away from the LED device due toinadequate heat conduction of the luminaire housing, which may beexacerbated in a hot environment.

Although LED package designs include heat carriers and heat sinks thatconduct heat away from the LED device, it is nevertheless advantageousto determine the temperature of the LED device in order to takecorrective measures if heat is not dissipated sufficiently to maintainthe temperature of the LED device below a critical level. A conventionalway to determine the temperature of the LED device is to place athermistor or thermocouple on the LED package near the LED device.However, this method does not measure the temperature directly at theLED dies covered by the transparent carrier material. Depending on howthe heat propagates away from the LED dies, the temperature at thethermistor does not reflect the actual temperature under the transparentcarrier material. Moreover, this manner of measuring temperatureprovides a relatively slow feedback and can lead to oscillation in thetemperature control. Because the source of the heat is the LED dies andthe phosphor particles under the transparent carrier material, thetemperature at the thermistor or thermocouple outside the transparentcarrier material is indicative of the heat that was produced earlierwithin the transparent carrier material. By the time the thermistor orthermocouple measures a temperature that exceeds a threshold and LEDdrive current is reduced in order to reduce the heat generated by theLED device, the temperature within the transparent carrier material mayalready have fallen because the temperature measured at the thermistoror thermocouple resulted from earlier produced heat that later reachedthe thermistor or thermocouple. The delayed feedback will cause thecurrent control to overcompensate both after the measured temperatureexceeds an upper threshold and after the measured temperature fallsbelow a lower threshold. An oscillating LED device temperature results.

Thermistors and thermocouples are typically not placed near the LED diesunder the transparent carrier material, however, because they absorblight and would result in a non-uniform pattern of light generation fromthe LED device. Moreover, placing a thermistor or thermocouple withinthe LED array would add an additional manufacturing step and wouldrequire additional machinery. So the cost of the resulting LED devicewould increase significantly. A inexpensive method is sought fordetermining the temperature of LED dies covered by a transparent carriermaterial that includes phosphor without causing the light emitted fromthe LED device to be non-uniform.

SUMMARY

A light-emitting diode (LED) device includes first and second LED diesthat both have the same structure and that are both encapsulated by asilicone layer. Driver circuitry supplies the first LED die withsufficient drive current to illuminate the first LED die. Controlcircuitry supplies the second LED die with a constant current,determines the voltage across the second LED die, and calculates thetemperature of the second LED die based on the voltage across the secondLED die. The LED drive current has a maximum magnitude that exceeds tenmilliamps, and the constant current that is supplied to the second LEDdie never exceeds ten milliamps. Thus, the maximum magnitude of theconstant current never exceeds the maximum magnitude of the drivecurrent. The LED device is able to calculate the temperature of a diodewith a gallium-nitride layer (GaN or GaInN) that is receiving a largedrive current and emitting blue light by determining the voltage acrossan adjacent similar diode with a gallium-nitride layer through which asmall constant current is flowing.

A method for determining the temperature of an LED die of an array ofLED dies covered by a silicone layer involves determining the voltagedrop across a single LED die. Both a first LED die and a second LED dieare encapsulated by the same silicone layer in which phosphor particlesare suspended. And both the first LED die and the second LED die havethe same structure. In one implementation, the band gap of the LED diesexceeds two electron volts. The first LED die is illuminated bysupplying a drive current to the first LED die. While the second LED dieis being supplied with a small constant bias current, the voltage acrossthe second LED die is determined. The temperature of the second LED dieis determined based on the voltage across the second LED die.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a top view of a device with an LED die used to determine thetemperature under a silicone layer that covers an array of other LEDdies.

FIG. 2 is a cross-sectional view of a device that uses an LED die todetermine the temperature within a silicone layer and roughlycorresponds to a cross section of the device of FIG. 1.

FIG. 3 is another cross-sectional view of a device that uses an LED dieto determine the temperature within a silicone layer and roughlycorresponds to another cross section of the device of FIG. 1.

FIG. 4 is a graph of the temperature-voltage relationship of agallium-nitride diode under a constant current of five milliamps.

FIG. 5 is a graph of the temperature-current relationship of agallium-nitride diode under a constant voltage of 2.6 volts.

FIG. 6 is a simplified schematic block diagram of control circuitry thatsupplies a sensor LED die with a constant current and that determines avoltage across the LED die.

FIG. 7 shows an embodiment of the device of FIG. 1 in which the controlcircuitry of FIG. 6 has been incorporated into the ceramic package.

FIG. 8 is a flowchart illustrating a method for determining thetemperature of an LED die of an array of LED dies covered by a siliconelayer.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 shows a light-emitting diode (LED) device 10 with an LED die 11used to determine the temperature under a transparent carrier materialthat includes phosphor and that covers an array of other LED dies. LEDdevice 10 includes an array of sixty-one structurally identical LED dies(chips). Each of the LED dies includes epitaxial layers of GaN or GaInNgrown on a sapphire substrate. In other embodiments, the gallium-nitridelayer is grown on a substrate of crystalline silicon. Each of thesixty-one LED dies is mounted on an aluminum substrate 12 that is housedin a ceramic package 13. The gallium-nitride LED dies emit blue lightwith a wavelength of about 452 nanometers when a sufficient drivecurrent is passed through the diodes. For example, a string of ten LEDdies 14-23 are connected in series such that a drive current can flowfrom a positive supply terminal 24 through the LED dies 14-23 to anegative supply terminal 25. The LED dies are connected to each otherand to the supply terminals by bond wires 24. For example, a bond wire26 connects LED die 14 to a landing pad 27 of positive supply terminal24.

The array of sixty-one LED dies is covered by a transparent carriermaterial, such as a layer of silicone or epoxy. Particles of phosphorare suspended in the transparent carrier material. The phosphor convertsa portion of the blue light generated by the LED dies into light in theyellow region of the optical spectrum. The combination of the blue andyellow light is perceived as “white” light by a human observer. A slurryof phosphor suspended in silicone is dispensed into a ring or dam 28around the array of LED dies. The silicone layer inside dam 28 forms anoptical surface, such as a lens or a structured surface. The blue lightfrom the LED dies and the yellow light from the phosphor particles ismore likely to exit the silicone layer if the surface is structured asopposed to smooth because the scattered light is more likely to strikethe surface at a normal angle that exhibits a lower total internalreflection (TIR). For example, the structured surface of the siliconelayer can have a small sinusoidal wave structures, “rectified” wavestructures (hemispheres) or saw-tooth structures.

Drive current is not passed through LED die 11, and LED die 11 is notconnected to the power supply terminals or other driver circuitry.Instead, sensor LED die 11 is connected to control circuitry thatsupplies LED die 11 with a constant current and that determines thevoltage across LED die 11. At a constant current flowing through LED die11, the voltage across LED die 11 depends on the temperature of LED die11. Because the silicon layer encapsulates LED die 11 as well as theother LED dies and all of the dies are in the same environment, thetemperature of LED die 11 is approximately the same as the temperatureof the other dies in its vicinity, such as dies 14-15. In fact, thetemperature of all of the sixty-one LED dies under the silicone layer isnearly the same and does not vary by more than a few degrees. Certainly,the temperature of LED die 11 is much closer to the temperature of theother LED dies than would be the temperature of a thermistor that is notcovered by the silicone layer and that is placed outside dam 19.

FIG. 2 is a cross-sectional view of an LED device 30 that uses LED die11 to determine the temperature within a silicone layer 31. Thecross-sectional view of LED device 30 in FIG. 2 roughly corresponds tothe cross section of LED device 10 through the string of LED dies 14-23in FIG. 1. Sensor LED die 11 and the other LED dies are mounted directlyto aluminum substrate 12 in a chip-on-board (COB) manner. The top ofaluminum substrate 12 includes various layers of dielectric and metal toform contacts and isolations. In some embodiments, the illuminated LEDdies 14-19 are electrically connected to power and driver circuitry, andsensor LED die 11 is electrically connected to control circuitry,through conductive layers at the top of aluminum substrate 12 with bondwires connecting to tap points on the conductive layers. In theembodiment of FIG. 2, however, the illuminated LED dies 14-19 areelectrically connected to power through bond wires 26, 32. FIG. 2 showsthe silicone layer 33 that contains phosphor particles 34. Silicone oranother transparent carrier material is poured into dam 28 and hardensforming a conformal covering over the LED dies. The silicone layer 33also protects the bond wires 32.

FIG. 3 is another cross-sectional view of LED device 30 passing throughLED die 11. The cross-sectional view of LED device 30 in FIG. 3 roughlycorresponds to the cross section of LED device 10 through LED die 11 inFIG. 1. Sensor LED die 11 is adjacent to illuminated LED dies 14-15 inLED device 30. Whereas the illuminated LED dies 14-19 are connected todriver circuitry and are provided with a drive current, sensor LED die11 is connected to control circuitry that supplies LED die 11 with asmall constant bias current. The maximum magnitude of the constant biascurrent never exceeds the maximum magnitude of the drive current thatilluminates LED dies 14-19. In fact, the constant bias current is verysmall and never exceeds ten milliamps. In a preferred embodiment, theconstant current supplied by the control circuitry to sensor LED die 11is about one milliamp. However, the constant bias current can even be onthe order of microamps. LED die 11 is connected to the control circuitryby bond wires 35-36. The control circuitry determines the voltage acrosssensor LED die 11 and then determines the temperature of LED die 11based on that voltage.

FIG. 4 is a graph of the temperature-voltage relationship of agallium-nitride diode under a constant current of five milliamps. Thevoltage across gallium-nitride LED die 11 while a constant current isflowing varies linearly with the temperature of LED die 11. The voltageacross LED die 11 decreases as the temperature increases while theconstant current is flowing. For example, the control circuitry candetermine that the temperature of LED die 11 is about 105° C. if thevoltage across LED die 11 is 2.45 volts while a constant current of 5 mAis flowing through LED die 11.

Although all of the LED dies under silicone layer 33 are capable ofgenerating light, they are nevertheless diodes and exhibit the standardcharacteristics of a diode. A diode is created by joining a p-typesemiconductor with an n-type semiconductor to form a pn junction. Thep-type semiconductor is doped with a trivalent atom such as indium oraluminum. The three valence electrons covalently bond with thesemiconducting material and leave a “hole” in the fourth bond. Then-type semiconductor is doped with a donor atom such as arsenic. Four ofthe donor atom's electrons bind covalently with the semiconductingmaterial while the fifth electron is free to move into the conductionband if the diode receives the appropriate amount of energy. The amountof energy required to move electrons into the conduction band is theband gap energy. The band gap of a standard silicon diode is 1.1electron volts, and the band gap of a red diode is about 1.4 electronvolts. The band gap of the gallium-nitride, light-emitting diodes in LEDdevice 30, however, is much higher. Gallium-nitride LED dies that emitblue light at about 452 nanometers have a band gap of 2.7-2.8 electronvolts.

The voltage across a diode through which a constant current is flowingvaries with temperature according to the relationship V=C−T/B, where Cis indicative of the constant current, and B is indicative of the bandgap energy of the diode. For a constant current of 5 mA, C equals2.5873. For a gallium-nitride diode that emits light at 452 nanometers,B equals 769.231. Thus, the temperature-voltage relationship shown inFIG. 4 can be expressed as V=2.5873−T/769.231. Using the voltage Vacross sensor LED die 11 as an input, the control circuitry calculatesthe temperature of LED die 11 using the formula T=769.231×(2.5873−V) fora constant current of 5 mA flowing through LED die 11. The calibrationfactor C must be adjusted when a different constant bias current isused.

Using a smaller constant bias current has the advantage that less heatis produced as current flows through sensor die 11. Any heat produced bythe bias current results in a higher temperature around sensor die 11that around the other LED dies. In addition, some 452-nm light isgenerated even by a small bias current. The blue light emitted by sensorLED die 11 even with a small bias current results in a color overposition inhomogeneity of the overall light emitted from LED device 30and should be minimized. However, a smaller constant bias current alsoresults in a lower signal-to-noise ratio of the voltage detection signalfrom sensor die 11. A good compromise between reducing heat and colorinhomogeneity and reducing noise in the temperature signal is a constantbias current of between 0.1 mA and 1 mA.

Silicon diodes and red diodes would be unsuitable for sensing thetemperature inside silicone layer 33 because these diodes would absorbthe longer wavelength light emitted by the phosphor particles 34 andwould produce a current. Just as light-emitting diodes produce lightwhen a current is passed through the diodes, the diodes produce acurrent when light with the appropriate amount of energy (the band gapenergy) is absorbed by the diodes. The current produced when light witha band gap energy of 1.1 or 1.4 eV for silicon or red diodes is absorbedwould add to the constant bias current, would effect the voltagedetection signal and would thus interfere with the temperaturemeasurement. On the other hand, white light and the light emitted by thephosphor particles 34 does not have sufficient energy to bridge the bandgap of the gallium-nitride, light-emitting diodes of LED device 30.Whereas diodes with a band gap energy of 1.1 or 1.4 eV would absorbalmost 100% of the light emitted by LED dies 14-19, gallium-nitride LEDdie 11 with a band gap energy of 2.7-2.8 eV absorbs only a fraction of1% of the light that strikes it within LED device 30.

The low light absorption of gallium-nitride LED dies compared to siliconor red diodes has another advantage besides not interfering with thetemperature measurement. The low light absorption of gallium-nitride LEDdies allows one of the dies to be used to sense temperature within thesilicone layer 33 without decreasing the lumen output of LED device 30.Because a silicon or red diode would absorb almost 100% of the generatedlight that strikes it, such a diode would have to be covered by areflective material to prevent absorption. The sapphire substrate of LEDdie 11, however, is substantially transparent to the white light.Placing a gallium-nitride LED die under silicone layer 33 and using thedie to sense temperature will not create a dark spot on LED device 30.

Other advantages of using a gallium-nitride LED die instead of a siliconor red die to sense temperature under silicone layer 33 are cost andperformance. Using diodes in LED device 30 that are all of the same typeis less expensive than sourcing and placing a second type of diode nextto the LED dies on substrate 12. The cost of placing an additional LEDdie on substrate 12 to be used to sense temperature is minimal becausethe same processes and equipment is used. The performance of LED die 11that is used as a temperature sensor is also superior to that of asilicon or red diode. The LED die 11 will last as long as the other LEDdies on LED device 30 that are of the same type. Moreover, the LED dieshave been designed and tested to last for 50,000 hours and to resistspikes in temperature of up to 200° C.

FIG. 5 is a graph of the temperature-current relationship of a GaN diodeunder a constant voltage. The temperature of LED die 11 can also bedetermined by monitoring the current through LED die 11 while thevoltage across LED die 11 is maintained constant. The curve of FIG. 5shows how the current flowing through LED die 11 varies with thetemperature of LED die 11 as a constant voltage of 2.6 volts ismaintained across LED die 11. However, this method is not preferred oversensing the voltage with constant current because the current flowingthrough a diode with a constant voltage does not vary linearly withtemperature. Consequently, each sensor LED die would have to becalibrated to determine the best-fitting curve through the calibrationpoints. For the particular GaN LED die that generated the calibrationpoints of FIG. 5, an approximate relationship between temperature andcurrent for a constant voltage of 2.6V across the diode can be expressedas I=7.3794 e^(0.02Γ). The non-linear relationship of temperature tocurrent also requires a more complex control circuitry to calculate thandoes the linear relationship of temperature to voltage.

FIG. 6 is a simplified schematic block diagram of control circuitry 40that supplies sensor LED die 11 with a constant current 41 and thatdetermines a voltage across LED die 11. Control circuitry 40 includes avoltage detector 42 that determines the voltage across LED die 11 usinga voltage detection signal 43. Voltage detector 42 includes adifferential amplifier, a band pass filter, an analog-to-digitalconverter and a microcontroller to read out the voltage value. Controlcircuitry 40 generates a constant current using constant currentgenerator 44. Constant current generator 44 includes an N MOSFET 45, anoperational amplifier 46 and a shunt resistor 47. In one embodiment,constant current generator 44 sinks a constant 1 mA current 41 throughLED die 11. The 1 mA current 41 is monitored by operational amplifier 46using resistor 47. Operational amplifier 46 compares the voltage acrossresistor 47 to a reference voltage and opens transistor 45 to the extentrequired to maintain the voltage across resistor 47 at the referencevoltage. The current flowing through transistor 45 that is required tomaintain the voltage across resistor 47 at the reference voltage is the1 mA current 41.

FIG. 7 shows an embodiment of LED device 10 in which the controlcircuitry 40 of FIG. 6 has been incorporated into ceramic package 13.LED device 10 includes a first LED die 14 and a second LED die 11 whichboth have the same structure. Both dies 11 and 14 are covered bytransparent carrier material 33 that encapsulates the dies. First LEDdie 14 is used to emit blue light, and second LED die 11 is used as athermocouple to sense temperature. An LED drive current with a magnitudesufficient to illuminate the first LED die 14 is supplied to the firstLED die 14 through landing pad 27 of positive supply terminal 24.Landing pad 27 and supply terminal 24 are part of the driver circuitrythat supplies the first LED die 14 with the drive current. The drivercircuitry includes a driver that is not incorporated into ceramicpackage 13. Control circuitry 40 supplies the second LED die 11 with aconstant current and determines the voltage across the second LED die11.

LED device 10 can be used to illuminate first diode 14 by supplying adrive current to first diode 14. At the same time, LED device 10 withcontrol circuitry 40 is used to supply second diode 11 with a constantcurrent whose maximum magnitude never exceeds ten milliamps and todetermine the temperature of second diode 11 based on the voltage acrosssecond diode 11. The temperature of both first diode 14 and second diode11 is the same because both are encapsulated by silicone layer 33 withsuspended phosphor particles 34. In one implementation, the maximummagnitude of the constant current flowing through second diode 11 isfive milliamp. The control circuitry calculates the temperature indegrees Celsius of second diode 11 based on the voltage across seconddiode 11 using the formula T=769.231×(2.5873−V). The voltage detector 42of control circuitry 40 outputs a temperature signal 48 that provides areal-time indication of the temperature of the LED dies in LED device10. Temperature signal 48 is provided to an integrated control module 49that can take action in the event that the temperature of the LED diesexceeds a predetermined threshold. For example, the integrated controlmodule can reduce the drive current to the LED dies to reduce thetemperature within the silicone layer 33. Or the integrate controlmodule can send a message via a wireless interface indicating that LEDdevice 10 has exceeded the predetermined threshold for a measured amountof time.

FIG. 8 is a flowchart illustrating steps 50-52 of a method fordetermining the temperature of an LED die of an array of LED diescovered by a silicone layer. In a step 50, a first LED die with agallium-nitride layer is supplied with a small constant bias current.All of the LED dies of the array have the same semiconductor structureas the first LED die. In one implementation, the maximum magnitude ofthe constant bias current never exceeds ten milliamps. In step 51, thetemperature of the first LED die is calculated by determining thevoltage across the first LED die. In step 52, a second LED die with agallium-nitride layer is illuminated by supplying the second LED diewith a drive current. Both the first LED die and the second LED die areencapsulated by a silicone layer. The method involves measuring avoltage drop across a single LED die that is covered by a silicone layeralong with other LED dies of the same structure.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Although the sensor LED die 11 is described above asbeing a GaN diode that emits light at 452 nanometers, an LED that emitslight at other wavelengths can also be used to sense the temperatureunder the silicone layer 33. For example, a GaInN diode or a diode thatdoes not contain gallium can be used. But the value for B in the formulaV=C−T/B must correspond to the band gap energy of the other diodeinstead of that of the 452-nm GaN LED. Accordingly, variousmodifications, adaptations, and combinations of various features of thedescribed embodiments can be practiced without departing from the scopeof the invention as set forth in the claims.

1-20. (canceled)
 21. A device comprising: a diode that has atemperature; an encapsulant that covers the diode, wherein theencapsulant contains phosphor; and circuitry adapted to determine thetemperature of the diode based on a voltage across the diode.
 22. Thedevice of claim 21, wherein the diode has a gallium nitride (GaN) layer.23. The device of claim 21, wherein the diode has a band gap thatexceeds two electron volts.
 24. The device of claim 21, furthercomprising: a second diode, wherein the encapsulant covers the seconddiode, and wherein the second diode emits light.
 25. The device of claim24, wherein each of the diode and the second diode has a gallium nitride(GaN) layer.
 26. The device of claim 21, wherein the circuitrydetermines the temperature of the diode by supplying a constant currentto the diode.
 27. The device of claim 26, wherein the constant currentnever exceeds ten milliamps.
 28. A device comprising: a first diode; anencapsulant through which light is emitted, wherein the encapsulantcovers the first diode, and wherein the encapsulant has a temperature;and circuitry adapted to use the first diode to determine thetemperature of the encapsulant.
 29. The device of claim 28, wherein thefirst diode has a gallium nitride (GaN) layer.
 30. The device of claim28, wherein the first diode has a band gap that exceeds two electronvolts.
 31. The device of claim 28, wherein the encapsulant containsphosphor.
 32. The device of claim 28, further comprising: a seconddiode, wherein the encapsulant covers the second diode, and wherein thesecond diode emits the light.
 33. The device of claim 32, wherein eachof the first diode and the second diode has a gallium nitride (GaN)layer.
 34. The device of claim 32, wherein the first diode and thesecond diode have the same structure, and wherein the second diode emitslight with a wavelength between 445 and 455 nanometers.
 35. The deviceof claim 28, wherein the circuitry determines the temperature bysupplying a constant current to the first diode and by sensing a voltageacross the first diode.
 36. The device of claim 28, wherein the firstdiode is supplied with a current that never exceeds ten milliamps.
 38. Adevice comprising: a first diode that has a temperature; a second diode,wherein each of the first diode and the second diode has a galliumnitride (GaN) layer; an encapsulant that covers the first diode and thesecond diode; and circuitry adapted to determine the temperature of thefirst diode based on a voltage across the first diode.
 39. The device ofclaim 38, wherein the circuitry determines the temperature of the firstdiode by supplying a constant current to the first diode.
 40. The deviceof claim 39, wherein the constant current never exceeds ten milliamps.